Scientists are learning that
the key to predicting certain epidemics -- like Rift Valley fever
in Africa or Hanta virus in the U.S. -- lies in an unexpected
place: the ocean.

April 17,
2002: On the dusty savannahs
of eastern Africa, where livestock sustain the economy, about
twice a decade an epidemic whips through to decimate the herds.
Nearly all of the pregnant animals spontaneously lose their fetuses.
Among those already born -- the lambs and kids -- the mortality
rate can reach 90 percent.

It's called Rift Valley fever.

Humans can be infected as well, either
through mosquitoes that carry the disease, or by handling infected
tissue. Few die, but the illness can cause serious complications:
meningoencephalitis, an inflammation of the brain, and lesions
of the retina, which leave victims with at least some permanent
loss of vision. During the most recent and devastating outbreak
in 1997-98, an embargo banned exports of East African meat for
one and a half years.

Above: Rift Valley fever was
first identified during the 1930's when it struck residents of
Kenya's great Rift Valley, pictured here. [more]

While no easy treatment exists
for the disease, Rift Valley fever can be controlled. Animals
can be vaccinated, insecticides can be spread into the soil to
keep infected mosquitoes from hatching. But the disease's unpredictability
has been a sticking point: without knowing when and where the
disease will strike, it's hard to know how to use those controls
efficiently.

But the disease might not be so capricious
after all, says Assaf Anyamba of NASA's Goddard Space Flight
Center.

Anyamba and colleagues at Goddard and
at the Walter Reed Army Institute of Research have discovered
that outbreaks of Rift Valley fever follow sudden floods triggered
by El Niño and a similar (yet lesser-known) climate disturbance
called the "Indian Ocean Dipole." Using
weather satellites to track sea surface temperature patterns
in the Indian and Pacific oceans, they now believe they have
found a way to predict outbreaks up to five months in advance.

Sea surface temperatures can predict
the likelihood of the disease because tiny variations in these
temperatures cause huge shifts in air circulation patterns --
shifts that alter rainfall around the globe. El Niño, for example, happens when a band of warmer-than-average
water forms near the Pacific coast of South America. Meanwhile,
Pacific waters near Australia and Indonesia become a bit cooler
than usual.

Above: This false-color map
of sea surface height (which is related to surface temperature)
reveals the signature of El Niño in the Pacific Ocean
and a similar imbalance in the Indian Ocean. Image credit: Tony
Phillips. [more]

A similar type of temperature imbalance
can occur in the Indian Ocean, with the western part near Africa
becoming warmer than the eastern part near Australia. Indeed,
researchers liken this "Indian Ocean Dipole" to El
Niño in the Pacific. Both tend to increase rainfall
in East Africa.

When the two anomalies occur at the
same time, buckets pour.

"The year 1997 saw the largest
El Niño ever recorded simultaneous with a very large
Indian Ocean Dipole (see figure)," says Christina Clark,
an atmospheric scientist at the University of Colorado. "East
African rainfall was then the highest on record, in many places
five times the normal amount." Such floods bring Rift Valley
fever because water collects in shallow depressions called "dambos"
that punctuate the savannahs, providing mosquito eggs with exactly
the nurturing conditions that they need to hatch.

Tracking
sea surface temperatures isn't enough for disease forecasters,
however, because rainy weather isn't necessarily uniform across
the region. To pinpoint specific vulnerable areas, forecasters
look for the color green: "Where it rains there will be
an increase in greenness of vegetation," says Anyamba. Earth-orbiting
satellites that scan for verdant areas can spot places within
the savannahs where mosquitoes will thrive.

Right: This map shows the
greening of East Africa during an Indian Ocean warm spell in
1983. [more]

The data about both sea surface temperature
and vegetation is provided by NOAA's Advanced Very High-Resolution
Radiometer (AVHRR) -- a type of visible-light and infrared sensor
carried on many polar orbiting weather satellites. "These
satellites were not really designed to monitor land surface conditions,"
says Anyamba. "They were designed to monitor atmospheric
conditions -- basically, clouds." But Goddard scientist
Compton Tucker realized, says Anyamba, that by manipulating the
information provided by the AVHRR, he could produce a 'greenness
index,' which measured the condition of the vegetation on the
ground.

Using sea surface temperatures to predict
when East Africa might be vulnerable, and using the greenness
index to pinpoint exactly where, researchers can alert health
officials to potential danger. "What we can do is provide
public health officials with an efficient way of being able to
focus their resources, rather than sending teams out all over
the place," says Anyamba.

The method used to forecast Rift Valley
fever could be expanded to predict other types of epidemics.
It could be applied in areas that, like East Africa, are usually
dry, but sometimes experience heavy rains, and that, like East
Africa, have ecosystems that burgeon when precipitation comes.

Hanta virus outbreaks in the American southwest
could be monitored in this way. The virus is carried by deer
mice and can kill people who have been exposed to it. Like Rift
Valley fever, says Anyamba, Hanta virus is correlated with rainfall.
"The US southwest is really a very dry environment,"
he says, "and you are likely to see there the same kind
of bioclimatic rhythms that you see in East Africa."

Left: Deer
mice
and mosquitoes are disease vectors
that can suddenly flourish in areas made wet by El Niño.

Right now, says Anyamba, "we're
in operational mode [for East Africa]." Every month, he
and his colleagues post their findings on the web, so that people
in the field can "check the animals, check the people, see
whether there's any activity."

Things are quiet -- for now.

But sea temperatures will shift again.
And when they do, the work of Anyamba and his colleagues will
save lives.

More Information

Tracking
Disease from Space -- (VisibleEarth) Using weather satellites to
spot the early signs of El Niño, scientists may be able
to save East Africans and their livestock from Rift Valley fever,
a mosquito-borne disease that can be fatal to humans and animals.